epoll 事件 epollin 发生，第一次recv 是EAGAIN？

非阻塞socket的方式下，EPOLLIN事件并不一定表示有数据，recv的man手册上提到：If no messages are available at the socket, the receive calls wait for a message to arrive, unless the socket is nonblocking (see fcntl(2)), in which case the value -1 is returned and the external variable errno is set to EAGAIN or EWOULDBLOCK.

errno为EAGAIN继续等下一个事件处理即可，具体可看这里：

nonblocking socket recv problem while using it with epoll